1. Field of the Invention
The present invention relates generally to a radio frequency interference (RFI) filter. More particularly, the present invention relates to an RFI filter feed-through assembly which may be incorporated in an RF power amplifier.
2. Description of Related Art
Modern wireless communication base stations transmit and receive radio frequency (xe2x80x9cRFxe2x80x9d) signals through the use of RF power amplifiers. Within the amplifier assemblies are typically plural printed circuit (xe2x80x9cPCxe2x80x9d) boards on which components that process the RF signals are mounted. RF signals must be transmitted between the various processing components. These intra-device transmissions may be accomplished through the use of microstrip technology, such as coplanar waveguides.
Other direct current (xe2x80x9cDCxe2x80x9d) or low-frequency signals may coexist with the high frequency signals in the amplifier assembly. For some applications, it is necessary to isolate a PC board or a region of a PC board from interference by outside RF signals or intra-device RF signals, while at the same time providing for the communication of DC or low-frequency signals. For example, modern RF power amplifiers typically have control systems which provide monitoring and control through use of microprocessors and/or other control integrated circuits (xe2x80x9cICsxe2x80x9d). Therefore, the DC signal may be the control output of an IC.
Isolation from RF signals propagating through different sections of an assembly may be provided by a metal enclosure or shield. For instance, referring to FIGS. 1A and 1B (top plan view) which show a conventional construction of an RFI filter, one might may isolate two PC boards 71, 72 by placing them on opposite sides of a grounded metal barrier wall or bulkhead 4 within the assembly. The metal bulkhead 4 separates two regions 61, 62 lying on opposite sides thereof and shields electromagnetic radiation from being transmitted between the regions 61, 62. Communication of DC signals between the boards 71, 72 can be provided by an insulated conductive feed-through 3 which passes through the bulkhead 4. The feed-throughs 3 are mounted through the bulkhead 4 so that the conductive input and output terminals 20a, 20b lie on opposite sides of the bulkhead 4. Attached via ribbon cables 1 near the edges of the two PC boards 71,72 are connectors 2 which are adapted to mate with the conductive terminals 20a, 20b of the feed-through 3. However, this configuration provides only incomplete isolation because RF signals may still propagate along the feed-through 3.
To prevent such RF propagation, it is conventional to incorporate an RFI filter in the conductive feed-through 3. In FIG. 2A, a circuit diagram for one embodiment of a. conventional filter is shown. Lying between an input terminal 20a and output terminal 20b of the filter 3 is a node 21. Attached to the node 21 is a grounded shunt capacitor 5. In the normal operation of the filter 3 an input signal containing high frequency (about 0.9 MHz and above) and low frequency components (about 50 kHz and below) is introduced at the input terminal 20a. The shunt capacitor 5 acts as a low-pass filter, providing a path to ground for undesirable co-propagating RF signals while allowing DC or low frequency signals to pass through the bulkhead 4. Thus, the output signal presented at the output terminal 20b primarily consists of the low frequency components of the input signal.
Another variation of an RFI filter is the Pi filter, illustrated in FIG. 2B. Lying in series between the input terminal 20a and output terminal 20b of the feed-through 3 is an inductor 7. On opposite ends of the inductor 7 are nodes 21a, 21b. Attached at the nodes 21a, 21b are grounded shunt capacitors 5. In the normal operation of the filter 3, an input signal containing high frequency and low frequency components is introduced at the input terminal 20a. The shunt capacitors 5 provide a path to ground which substantially attenuates the high frequency components of the input signal. Moreover, the inductor 7 presents a high impedance to high frequency components and provides additional attenuation of the high frequency components. Thus, the output signal presented at the output terminal 20b primarily consists of the low frequency components of the input signal. The feed-through filters described in FIGS. 2A and 2B provide bi-directional filtering of high frequency components, i.e. an input signal may be alternatively introduced at the output.
A drawback to conventional feed-through RFI filters is that their use entails added cost. First, the feed-through component 3 itself is costly. Moreover, since the conductive leads 20a, 20b of the feed-through component 3 are usually not directly coupled to the PC boards 71, 72 on either side of the bulkhead 4, the PC boards 71, 72 must somehow be joined to the conductive leads 20a, 20b of the feed-through 3. As shown in FIGS. IA and 1B, this is conventionally done by attaching to each PC board 71, 72 a connector 2 which mates with the feed-through leads 20a, 20b and connects to PC board leads 1. The use of a separate connector adds to the manufacturing cost of an RF assembly, and may increase the labor cost if the connector on the PC board must be manually mated with the leads of the feed-through component. Also, the feed-through components and separate connectors conventionally used are bulky.
Accordingly, a need presently exists for effective prevention of RF interference between sections of an RF assembly that is less expensive than prior art solutions.
An RFI filter assembly is provided which overcomes the deficiencies of the prior art.
In one aspect, a low cost RFI filter assembly is provided which comprises a multi-layered printed circuit board assembly. In a preferred embodiment, the layered PC board assembly is adapted to pass below or through a shield bulkhead, or partition, for conducting DC or low frequency signals through the shield bulkhead. In a preferred embodiment, three conductive layers are provided, separated by non-conductive insulation layers. A top layer includes a conductive ground plane. A middle trace layer of the three-layered embodiment comprises several narrow conductive paths that traverse the length of the board surrounded by grounded regions. The narrow conductive paths function as an inductor which resists the propagation of high-frequency signals from one end of the board to the other. A bottom layer of the three-layered embodiment comprises a ground plane. The grounded regions of all three layers are electrically coupled by plated ground via holes through all three layers.
In a further aspect of the invention, affixed to the ground plane are two groups of capacitive elements spaced across the width of the top ground plane on opposite sides of the bulkhead. Each of the capacitive elements comprises two conductive contacts One contact of each capacitive element in the first group is electrically coupled to the top ground plane while the other contact is electrically coupled to a respective conductive path traversing the middle trace layer of the board assembly. The second group of capacitive elements is similarly coupled at the opposite end of the board. The capacitive elements provide a shunt path for high-frequency signals to ground, and thus attenuate high-frequency signals introduced at the extreme ends of the conductive paths.
This configuration functions as a Pi filter providing effective filtering of RF signals without the use of bulky feed-through components. Also, ground layers may be provided on the sides of the narrow conductive paths. These prevent RF radiation propagating through the assembly. Therefore, RF interference between shielded regions is effectively blocked.
The invention, now having been briefly summarized, may be better appreciated by the following detailed description.